Measure Guideline: 

Combustion Safety for Natural 

Draft Appliances Using Indoor 

Air

L. Brand 

Partnership for Advanced Residential Retrofit 

April 2014
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iii

Measure Guideline: Combustion Safety for Natural Draft 

Appliances Using Indoor Air

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

L. Brand 

Partnership for Advanced Residential Retrofit

Gas Technology Institute

1700 S. Mount Prospect Road 

Des Plaines IL 60018 

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40346-03 

April 2014 
iv

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v 

Contents

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Abstract..................................................................................................................................................... viii

Acknowledgments ................................................................................................................................... viii

Progression Summary............................................................................................................................... ix

1 Introduction........................................................................................................................................... 1

2 Measure Implementation—Appliance Assessment .......................................................................... 3

2.1 Appliance Draft and Venting System Type Identification ............................................................... 4

2.2 Vent Inspection ................................................................................................................................ 5

2.3 Indoor Combustion Air Code Compliance ...................................................................................... 9

2.3.1 Standard Method—Required Volume ................................................................................ 9

2.3.2 Known Air Infiltration Rate Method ................................................................................ 10

3 Inspection and Test-In ....................................................................................................................... 11

3.1 Vent System and Chimney Inspection ........................................................................................... 11

3.2 Vent Performance and Combustion Testing .................................................................................. 12

3.2.1 On Entry and With Equipment Off ................................................................................... 12

3.2.2 Combustion Appliance Tests With Equipment On ........................................................... 13

4 Upgrade ............................................................................................................................................... 16

5 Test-Out............................................................................................................................................... 17

6 Estimate Remediation Cost............................................................................................................... 18

6.1 Estimate Remediation Cost at Code Compliance Inspection ......................................................... 18

6.2 Estimate Remediation Cost at Test-In ........................................................................................... 18

6.3 Estimate Remediation Cost at Test-Out ......................................................................................... 18

7 Perform Remediation Where Required ............................................................................................ 19

7.1 Standard Method Remediation ...................................................................................................... 19

7.2 Known Air Infiltration Rate Method Remediation ........................................................................ 19

7.3 Test-in Remediation ....................................................................................................................... 19

7.4 Test-Out Remediation .................................................................................................................... 20

7.5 Detail on a Sample of Remediation Options .................................................................................. 20

7.5.1 Installing Louvers in Doors That Separate the Appliances From Other Spaces in the 

House ................................................................................................................................ 21

7.5.2 Sealing Return Air Ducts and the Furnace Cabinet .......................................................... 21

7.5.3 Supplying Makeup Air as Required by the Code (NFPA 54 Section 9.3.1.5) .................. 23

7.5.4 Adding Combustion Air Openings to the Outdoors .......................................................... 23

7.5.5 Converting From Exhaust-Only Ventilation to Balanced Ventilation .............................. 23

7.5.6 Providing an Engineered Solution .................................................................................... 23

8 Materials for Duct Sealing ................................................................................................................. 24

References................................................................................................................................................. 25
vi 

List of Figures

Figure 1. Draft hood-equipped appliance................................................................................................. 4

Figure 2. Fan-assisted appliance .............................................................................................................. 4

Figure 3. Common venting two draft hood-equipped appliances.......................................................... 5

Figure 4. Common venting a draft hood-equipped water heater with a fan-assisted furnace into a 

Type B common vent ........................................................................................................................... 5

Figure 5. Example gas furnace nameplate ............................................................................................... 6

Figure 6. Masonry chimney liner dimensions with circular equivalents............................................... 6

Unless otherwise noted, all figures were created by PARR. 

List of Tables

Table 1. Appliance Assessment Job Breakdown .................................................................................... 3

Table 2. Vent Connector Capacity in Relation to Chimney Height ........................................................ 8

Table 3. Inspection and Test Job Breakdown........................................................................................ 11

Table 4. CO Thresholds............................................................................................................................ 14

Unless otherwise noted, all tables were created by PARR. 
vii 

Definitions

ACH Air changes per hour

AGA American Gas Association

ANSI American National Standards Institute

ASTM American Society for Testing and Materials

BPI Building Performance Institute

CO Carbon monoxide

HVAC Heating, ventilation, and air conditioning

IFGC International Fuel Gas Code

IMC International Mechanical Code

IRC International Residential Code

NFGC National Fuel Gas Code

NFPA National Fire Protection Association

NREL National Renewable Energy Laboratory

RESNET Residential Energy Services Network

UL Underwriters Laboratories
viii

Abstract

This Measure Guideline provides guidance on combustion safety inspection procedures for 

appliances and heating equipment that use indoor air for combustion and dilution of flue gases in 

low-rise residential buildings. Appliances of interest in this Guideline include Category I and II 

gas-fired residential furnaces and other natural draft appliances that are not represented by a 

code-based categorization approach for venting at this time. Only natural draft (i.e., nonpositive 

vent static pressure) gas-fired combustion appliances installed in the living space or in an area 

freely communicating with the living space, vented alone or in tandem with another appliance 

are considered here. A separate Measure Guideline addresses combustion appliances located 

either within the living space in enclosed closets or side rooms or outside the living space in an 

adjacent area such as an attic or garage that use outdoor air for combustion and venting. 

This document is for building performance inspectors and auditors working in homes where 

energy upgrades are being conducted. Advanced steps are provided for auditors with special 

traning or trained heating and cooling technicians. Normal operation of combustion appliances 

can be affected by building alterations whether or not air infiltration control is included in the 

package of measures being applied. 

The information provided here is based on requirements provided in several of the codes and 

echoed in the test procedures used by building performance testing agencies, including 

inspection of the vent, creating a depressurized environment that may impact natural draft 

appliances, and conducting tests to determine excessive spillage. The prescriptive guildelines in 

the code are also covered as a reference. The Guideline allows trained residential energy retrofit 

inspectors to confirm that combustion appliances can operate as expected, or provides guidance 

so the inspector can refer problem areas to a qualified technician. 

This document does not provide sufficient information to assure code compliance and is not a 

guideline for repair or modification of mechanical systems requiring a qualified contractor or 

service person.

Acknowledgments

This Guideline is the product of a collaborative effort. The author acknowledges the funding and 

support of the U.S. Department of Energy’s Building America Program. Special thanks to Jim 

Fitzgerald, Minnesota Center for Energy and Environment; Paul Cabot and Ted Williams, 

American Gas Association; and Paul Francisco, Illinois Sustainable Technology Center.
ix

Progression Summary

COMBUSTION 

APPLIANCE 

ASSESSMENT

ISOLATED CAZ 

WORK SCOPE

CAZ ISOLATION

FIELD 

CONFIRMATION

APPLIANCE 

ASSESSMENT

Appliance Assessment. Determine that the Category I appliances using indoor air 

for combustion freely communicate with the indoor environment. (Advanced Step) 

Also determine that the appliances meet code requirements for indoor combustion 

air, the vent system is properly sized, and that both are operating safely.

1

TEST-IN 

Inspection and Test. Inspect the vent system for corrosion or blockage and 2 perform spillage and combustion tests. 

REMEDIATION

Remediation Options. (Advanced Step) Where required, add combustion air 

openings to larger indoor spaces, provide mechanical combustion air supply, 

provide combustion air ducts, provide an engineered solution, or replace with 

direct vent appliances. 

6

TEST-OUT

Test-Out. Perform a standard combustion safety test procedure to ensure that 

upgrade measures do not impact combustion safety. If the gas appliances fail 4 combustion safety tests, proceed to Estimate Remediation Cost. 

! !

Do Not Proceed if:

• Combustion appliances are not operating safely.

• Vent system spills for more than 5 minutes after startup

INDOOR AIR 

FOR 

COMBUSTION

Combustion Safety When Using Indoor Air for Combustion. The purpose of this 

Guideline is to provide minimum requirements and recognized test procedures for evaluating 

safe installation and operation of combustion appliances in homes undergoing energy 

efficiency upgrades where indoor air is used for combustion and venting. Recommendations 

for remedial measures are provided where necessary.

UPGRADE

3 Upgrade. Perform upgrade or retrofit measures.

REMEDIATION

COST ESTIMATE

Cost Estimate. (Advanced Step) Where remediation is required, determine work 

scope for proper venting of combustion gases or provide an alternate solution. 

Compare job cost to other combustion safety approaches such as replacement 

with power or direct vent appliances.

5
1 

1 Introduction

This Measure Guideline covers how to assess and 

carry out combustion safety procedures for 

appliances and heating equipment that use indoor 

air for combustion and dilution of flue gases in 

low-rise residential buildings. Only Category I 

and II gas-fired residential furnaces and other 

natural draft appliances that are not represented 

by a code-based categorization approach for 

venting at this time are addressed. Only natural 

draft (i.e., nonpositive vent static pressure) gasfired combustion appliances installed in the living 

space or in an area freely communicating with the 

living space, vented alone or in tandem with 

another appliance are considered here. Appliances 

installed in a zone isolated from the indoor air in 

enclosed closets, side rooms, or outside the living 

space in an adjacent area such as an attic or 

garage that use outdoor air for combustion and 

venting are covered in a separate Measure 

Guideline (Fitzgerald and Bohac 2013). 

This Guideline applies to Category I and II appliances only and largely focuses on Category I 

appliances using atmospheric venting systems that employ a draft hood. According to American 

National Standards Institute (ANSI) z223.1/National Fire Protection Association (NFPA) 54 

(NFPA 2012a), a Category I appliance is defined as an appliance that operates with a nonpositive 

vent static pressure and with a vent gas temperature that avoids excessive condensate production 

in the vent. The same source defines a Category II appliance as having a nonpositive vent static 

pressure and with a vent gas temperature that may cause excessive condensate production in the 

vent. The U.S. Department of Energy (DOE 2013) defines Category I appliances as having a 

flue-loss efficiency of less than 83% and operate under negative pressure in the vent. Some 

Category I appliances rely on the natural draft of their hot combustion products for venting. 

Others rely on fans or blowers to overcome the flow resistance through a heat exchanger and 

push or pull the combustion products through the appliance, although the vent still operates 

under negative pressure. Residential draft hood-equipped water heaters and boilers, though not 

categorized, are included in this Measure Guideline.

The current minimum code in this area is the ANSI z332.1 National Fuel Gas Code (NFGC), 

which is also published by the National Fire Protection Association as NFPA 54. The 

International Code Council includes the same language in the International Fuel Gas Code 

(IFGC) (ICC 2013). For simplicity these codes will be referred to as the “NFGC and IFGC” 

except where specific sections of the code are cited. The inspector/auditor should have a good 

understanding of the minimum code to use this Measure Guideline. 

The purpose of this Measure 

Guideline is to provide inspection 

guidelines for a subset of houses 

where the combustion appliances are 

in the living space or communicating 

freely with the living space. The goal 

is to allow energy retrofits including 

tightening and changes to distribution 

and ventilation systems to proceed, 

while verifying that the upgrades did 

not negatively impact combustion 

safety. A list of possible remedial 

measures, if required, is provided in 

this document. 

1. Provide a guide to the general 

code requirements and inspection 

procedures that allow natural 

draft appliances to comply with 

combustion safety guidelines.

2. Provide some remediation 

options where these appliances 

fail the combustion safety test.
2 

Building Performance Institute (BPI 2012) and Residential Energy Services Network (RESNET) 

(RESNET 2010) provide test procedures that go beyond the minimum code. If the auditor is BPI 

or RESNET certified, those test procedures should be consulted. 

This Guideline: 

• Defines the screening protocol in the codes for Category I atmospheric appliance 

combustion safety 

• Describes the inspection procedure summarized from the codes and building performance 

inspection procedures 

• Describes remediation measures, where necessary, including some illustrative examples.
3 

2 Measure Implementation—Appliance Assessment

This section of the Guideline provides an assessment tool for the inspector to determine if the 

vent size and combustion air openings are adequate for the installation. The procedure in Section 

2.1 should be conducted at minimum. Inspectors skilled in vent sizing and combustion air 

opening analysis should also perform Sections 2.2 and 2.3. 

The goals of the appliance assessment are to determine if using indoor air for combustion is: 

1. Applicable for the types of combustion appliances that are being used in the house 

2. Compatible within the elements, structure, and conditions of the building 

3. Cost-effective compared to alternative methods, e.g., replacement with power-vented or 

direct vent appliances. 

The assessment will determine if the appliance installation meets minimum requirements, or for 

advanced inspections, meets code prior to conducting any combustion safety testing. Table 1

shows the job breakdown for the appliance assessment.

Several computer-based software packages and applications are available for vent system 

inspection. 

Table 1. Appliance Assessment Job Breakdown

Important Steps Key Points Reasons

1. Appliance Draft 

and Venting 

System Type 

Identification

Natural draft and/or fan-assisted Category I 

and II appliances are located in a space that 

is freely communicating with the 

conditioned space. 

Determine if 

Measure Guideline 

can be applied.

2. Vent Inspection

Determine that vent connector(s) and 

common vents are sized and installed 

according to either the NFGC, Chapters 12 

and 13 (NFPA 2012), or the IFGC Chapter 5 

(IRC 2013). 

Determine need for 

mechanical 

contractor and work 

required. 

3. Indoor 

Combustion Air 

Code Compliance

Determine if indoor combustion air meets 

the requirements of the Standard Method or 

Known Infiltration Rate Method either in the 

NFGC Section 9.3 or the IFGC, Section 340. 

Ensure code 

requirements are 

being met before 

work begins. 
4 

2.1 Appliance Draft and Venting 

System Type Identification

Vent systems are designed to carry products of 

combustion (mostly carbon dioxide and water 

vapor) away from the appliance to the 

outdoors. If a vent system fails, flue gases may 

enter the indoor space and create a hazard, 

including unsafe carbon monoxide (CO) 

levels. There are provisions in the NFGC and 

IFGC to mitigate the combustion safety hazard 

and those steps are provided later in this 

report.

For this Measure Guideline, the focus is on 

natural draft appliances. Fan-assisted Category 

I and II appliances are also an important 

consideration where they are common vented 

with natural draft appliances. Natural draft 

appliances are sometimes referred to as draft 

hood-equipped appliances or atmospheric 

appliances. Draft hoods are included in these 

designs to separate the vent system from the 

appliance burner to decouple the outdoor 

weather conditions from burner performance. 

Installation of these appliances is covered by 

the NFGC and IFGC and become code when 

adopted by the authority having jurisdiction. 

Category III–IV appliances are installed 

according to the manufacturers’ installation 

instructions and are not a part of this Measure 

Guideline. 

Category I appliances (water heaters, furnaces, 

and boilers) are used singly in homes or in 

combination (common vented). They are 

installed using Type B vent systems or 

masonry chimneys according to the NFGC and 

IFGC with Type B or single wall vent 

connectors. These appliances can have a draft 

hood (Figure 1) or a fan-assisted combustion 

system (Figure 2), they can be vented alone (Figure 1 and Figure 2), or can be common vented 

with one or more other Category I appliances (Figure 3 and Figure 4). Residential water heaters 

and boilers are not formally categorized, but the NFGC and IFGC provide guidance for venting 

draft hood-equipped residential water heaters and boilers in single appliance or in common vent 

mode. 

Figure 1. Draft hood-equipped appliance

Source: American Gas Association (AGA) and NFPA. 

ANSI Z223.1 NFPA 54 NFGC 2012 Edition. 

Figure 1.1 p. 54-162 (p. Z223.1-162).

Figure 2. Fan-assisted appliance

Source: AGA and NFPA. ANSI Z223.1/ NFPA 54 

NFGC 2012 Edition. Figure 1.2 p. 54-162 

(p. Z223.1-162).
5 

For this initial assessment, we need to 

confirm that:

1. The appliances have a draft hood (or 

draft diverter) and are labeled Category I 

on the manufacturers nameplate (CAT I 

in Figure 5). Appliances that use air from 

outdoors and vent directly outdoors or 

appliances with mechanical or 

engineered venting systems are not 

Category I appliances. 

2. The appliances use indoor air for 

combustion and venting. 

3. The appliances are installed in a space 

that freely communicates with the indoor 

environment of the home. Appliances in 

a ventilated closet that is sealed from the 

indoor space are addressed in a separate 

Building America Measure Guideline. 

Appliances in an unsealed attic, garage, 

or outdoor ventilated closet are 

considered to use outdoor air for 

combustion. 

Once this condition is satisfied, proceed to 

Section 2.2, Vent Inspection. Inspectors not 

skilled in vent sizing and combustion air 

opening sizing should proceed to Section 3. 

2.2 Vent Inspection

The objective of the vent inspection is to 

determine if the appliance vent system meets 

the minimum sizing guidelines in the code 

prior to performing combustion safety 

testing. If the vent or vent connector is 

corroded or disconnected it must be repaired 

(see Section 3 for guidance on repairs). For 

vent inspection: 

Advanced Steps (for those with proper training)

1. Inspect the vent connectors first, then the common vent or masonry chimney.

Figure 3. Common venting two draft hoodequipped appliances

Source: AGA and NFPA. ANSI Z223.1 NFPA 54 NFGC

2012 Edition. Figure 2.1 p. 54-163 (p. Z223.1-163).

Figure 4. Common venting a draft hoodequipped water heater with a fan-assisted 

furnace into a Type B common vent

Source: AGA and NFPA. ANSI Z223.1 NFPA 54 NFGC

2012 Edition. Figure 2.2 p. 54-163 (p. Z223.1-163).
6 

2. Make sure that vent connectors are 

sized according to Tables 3.1(a)–

3.2(i) in NFPA 54 with additional 

requirements for manifolds, offsets, 

three or more elbows, corrugated 

liners, etc. as listed in 13.1.2–13.2.30 

of the Code to ensure minimum 

performance. 

3. For draft hood-equipped appliances, 

determine the height of the vent 

connector above the draft hood 

opening (rise), the lateral distance of 

the vent connector before it enters the 

common vent (T or Y fitting), or the 

masonry chimney and the height of 

the common vent from the connector 

to the vent terminal above the roof. 

4. Note the diameter of the vent 

connector, usually 3, 4, or 5 in., the 

material (single wall or Type B), and 

the number of elbows in the 

connector. Note the diameter of the 

common vent. 

5. Verify that the slope of the vent 

connector is upward away from the 

appliance and no less than 1⁄2 inch per 

foot of length. 

6. Determine the appliance input rating 

from the nameplate (Figure 5).

7. Using the single-wall or Type B vent 

connector table, look up the height 

and connector rise row and move 

across to the vent connector diameter. 

The appliance input rate should be 

less than the NAT Max value for draft 

hood-equipped appliances and 

between the FAN Min and Max 

values for fan-assisted appliances. 

Repeat for all vent connectors. 

8. If all vent connector diameters are within range, continue to the common vent table and find 

the appropriate vent height row and follow across to the common vent diameter column. Add 

the input ratings from all common vented appliances together and determine if the value is 

Figure 5. Example gas furnace nameplate

Source: Gas Technology Institute

 

U.S. Manufacturing Company, Inc.

Certified as a forced air furnace CAT I

Equipped for use with natural gas at altitudes from 0 to 2000 ft. above 

sea level

Installation only in buildings constructed on site.

Model No. CSX123-4

Serial No. 1234567890

Power Supply 115 V

Natural Gas LP Gas

Heating Input Rating 100,000 80,000

Output Capacity 80,000 60,000

Temperature Rise 35 – 65 35 – 65

Figure 6. Masonry chimney liner dimensions with 

circular equivalents

Source: AGA and NFPA. ANSI Z223.1 NFPA 54 NFGC

2012 Edition. Figure 2.3 p. 54-165 (p. Z223.1-165). Table 

abbreviated.
7 

below the maximum value in the table. If all appliances are draft hood-equipped, use NAT + 

NAT, if all are fan assisted combustion systems, use FAN + FAN, and otherwise use FAN + 

NAT. If the vent connector or vent is not properly sized, replace it. The only exception to the 

rule is if the vent connector and/or common vent will be modified as a part of the upgrade 

process. If that is the case, the vent and vent connector need not be replaced, but the 

equipment must still pass the test-in requirements in the Section 3 and the test-out 

requirements in Section 5 of this guideline. 

9. For appliances using masonry chimneys, if the vent connector is not properly sized, identify 

the correct size using the procedure in Section 2.2 step 7 of this guideline with the exception 

that the internal area of the masonry chimney must be calculated (Figure 6). The combined 

capacity of the common vented appliances must be between the minimum and maximum 

capacity listed in the tables. If the chimney is too large, it should be relined with a metal liner 

or Type B vent. 

10. For external masonry chimneys, special rules apply. A metal liner may be required 

depending on the appliance input rate, the installation type, and the local 99% winter design 

temperature.

11. For vent connectors with more than two elbows, long lateral lengths beyond 18 in./in. of 

diameter, manifold offsets, or other geometries, the capacity in the tables is reduced. (See 

additional requirements for reductions for manifolds, offsets, more than two elbows, 

corrugated liners, and other modifications in Sections 13.1.2–13.2.30 of NFPA 54.) 

Once the vent system meets code, the adequacy of indoor air for combustion should be 

addressed. 

A 40,000-Btu/h natural draft water heater and an 80,000-Btu/h fan-assisted Category I furnace are 

installed. The appliances are common vented into a 20-ft. tall 6-in. diameter Type B common vent. 

The water heater uses a 4-in. vent connector and the furnace a 5-in. vent connector, both Type B. Both 

connectors have two elbows and a lateral length of 3 ft (see Table 2).

1. To check the water heater vent connector for compliance, follow the left column down to the 20-ft 

vent height.

2. Select the 2-ft vent connector rise row and follow it to the 4-in. vent connector column.

3. The capacity of the water heater is less than the NAT Max value of 66,000 Btu/h, so the 4-in. vent 

connector is acceptable. Note that the 3-in. vent connector with a maximum capacity of 37,000 Btu/h

would not be acceptable in this case.

4. To check the furnace, repeat steps 2 and 3 but follow the row to the 5-in. vent connector column.

5. The 80,000-But/h furnace capacity is between the FAN Min and FAN Max values (48,000–67,000), so 

this connector diameter is acceptable.

6. For the connectors, use of two elbows does not require a capacity adjustment, and a 3-ft lateral is 

within the 18 in./in. of diameter restriction (18 * 4 = 72 in. or 6 ft. for the water heater and 18 * 5 = 90 in. 

or over 7 ft for the furnace).

7. To check the common vent, in the Common Vent Capacity table at the bottom of Table 2, follow the left 

column down to the 20-ft vent height and follow that row across to the 6-in. vent diameter. The FAN + 

NAT maximum capacity is 255,000 Btu/h. The furnace plus water heater capacity is 120,000 Btu/h, so 

the common vent diameter is acceptable.
8 

Table 2. Vent Connector Capacity in Relation to Chimney Height

Source: AGA and NFPA. ANSI Z223.1 NFPA 54 NFGC 2012 Edition. Figure 2.3 p. 54-120 (p. Z223.1-120).
9 

2.3 Indoor Combustion Air Code Compliance

According to the NFGC and IFGC, there are two alternative paths for determining if combustion 

air from the indoors is adequate for the home: the Standard Method and the Known Air 

Infiltration Rate Method. Both methods calculate the minimum volume of the space required for 

the total appliance input rate. The Standard Method is to be used unless the air infiltration rate is 

known to be less than 0.4 air changes per hour (ACH) of natural infiltration measured without a 

blower door. If the natural infiltration rate exceeds 0.4 ACH, the Known Air Infiltration Rate 

Method is to be used. Where louvers or grilles are employed to allow adjoining space to 

communicate freely, follow the sizing guidelines and use the net free area of louvers and grilles. 

Additional detail regarding a combination of indoor and outdoor air, engineered installations, and 

supplying makeup air is provided in the NFGC and IFGC. 

Research is underway to investigate the impact of depressurization on using indoor air for 

combustion and dilution. Results will be included in the NFGC and IFGC or updated Measure 

Guidelines when they are available. 

2.3.1 Standard Method—Required Volume

The total required volume is calculated by adding the volume of the space in which the appliance 

is installed and rooms communicating directly with the space through openings not furnished 

with doors and through combustion air openings sized and located in accordance with NFPA 54 

Section 9.3.2.3. The minimum required volume in the Standard Method is 50 ft3

/1,000 Btu/h. 

Required Volumestandard = 50ft3 � I

1,000 Btu/h

�

Where I is the total input of all appliances in the space in Btu/h.

A 40,000-Btu/h Category I water heater has a required volume of 50 × 40 = 2,000 ft3. 

If the water heater is installed in a 20 ft × 20 ft open basement with an 8-ft ceiling, the volume of the space 

is 20 × 20 × 8 = 3,200 ft3

. 

The volume of the space is above the minimum and the Standard Method is satisfied. 

For a 40,000-Btu/h water heater and an 80,000-Btu/h furnace in the same basement space, the required 

volume is 50 ×120 = 6,000 ft3

 

The basement volume of 3,200 ft3

, above, would not meet the requirement. 

If there is a louvered door to a finished basement room or to the first floor of the home that meets the 

requirements of the NFGC and IFGC, that volume can be included in the calculation to meet the required 

volume.
10 

2.3.2 Known Air Infiltration Rate Method

If the natural air infiltration rate in the home is known to be less than 0.4 ACH, an alternate 

method is used. For other than fan-assisted combustion systems, the required volume is: 

Required Volumeother ≥

21 ft3

ACH � Iother1,000 Btu/h

�

Where Iother is the total input of all other than fan-assisted appliances in the space in Btu/h.

For appliances with fan-assisted combustion systems, the required volume is: 

Required Volumefan ≥

15 ft3

ACH � Ifan1,000 Btu/h

�

Where Ifan is the total input of all fan-assisted appliances in the space in Btu/h. 

A 40,000-Btu/h draft hood-equipped water heater in a house with 0.4 ACH infiltration rate requires a volume 

≥ 21/0.4 * (40,000/1,000) = 2,100 ft3

. 

If there is a louvered door to a finished basement room or to the first floor of the home that meets the 

requirements of the NFGC and IFGC, that volume can be included in the calculation to meet the required 

volume.

An 80,000-Btu/h fan assisted furnace in a house with 0.4 ACH infiltration rate requires a volume ≥ 15/0.4 * 

(80,000/1,000) = 3,000 ft3

. If the two appliances in the examples in this section were installed in the same 

house, the required volume is 2,100 + 3,000 = 5,100 ft3

. This value is slightly lower than the value in 

Section 2.3.1 because fan-assisted combustion systems require less air for combustion and dilution.
11 

3 Inspection and Test-In

In this step, the vent system and chimney are inspected for corrosion and blockage, and the 

performance of the vent system is verified. This step is required to provide a safe working 

condition for the upgrade at test-in, and to ensure combustion safety is met when performed at 

test-out. 

Table 3. Inspection and Test Job Breakdown

Important Steps Key Points Reasons

1. Vent System 

and Chimney 

Inspection

Inspect the vent system for proper sizing and 

then for corrosion, blockage, and gaps, and 

the masonry chimney for blockage, holes, 

and the presence of a liner if required.

Ensure that the vent system 

is in good working order. 

2. Building 

Exhaust 

Systems and 

Air System 

Inspection

Inspect exhaust fans, clothes dryers, and 

kitchen ventilation systems for provision of 

makeup air in accordance with NFGC 

Section 9.3.1.5 (or as required in operational 

testing and comformance to safe 

performance under that section), and Section 

504.5 of the IMC. Return air intakes and 

their locations shall conform to the 

requirements of IMC Section M1602. 

Ensure minimum code 

compliance to avoid known 

potential sources of house 

depressurization and 

interference with proper 

appliance venting.

3. Vent 

Performance 

Testing

Determine that the vent system is 

performing as designed.

Ensure a safe working 

environment at test-in and 

combustion safety when 

performed at test-out. Find 

and reduce interference 

from other house systems 

during operation. 

3.1 Vent System and Chimney Inspection

Visually inspect vent connectors, vertical vents and masonry chimneys for perforated corrosion, 

gaps, and missing sections (see BPI 1200 and NFPA 54 chapter 12). Chimneys should be lined in 

accordance with NFPA 211, Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning 

Appliances (NFPA 2012b). Relining may be required by the NFGC and IFGC if the capacity of 

the chimney is too large for the vented appliances. Evidence of liquid water in a vent connector, 

vent, below the chimney cleanout, or from the base of a chimney liner that penetrates indicates 

the need for repairs. Repairs should be completed before vent performance testing begins. 
12 

3.2 Vent Performance and Combustion Testing

The next step is to make sure that the home upgrade is taking place under safe working 

conditions. A combustion safety check shall be performed for each appliance. If for any reason 

the combustion appliances are deemed to be operating unsafely during this visit, the Measure 

Guideline should be abandoned and the homeowner advised that further inspection, repair, tuneup, or replacement may be necessary by an 

appropriately licensed technician. 

The procedure provided in this section is based on 

NFPA 54 Annex G with some modifications to CO 

testing and door positions to reflect changes being 

considered by NFPA and other organizations. 

Similar procedures are available from BPI and 

RESNET. These procedures are all undergoing 

revision at the time of this publication. Following 

this Measure Guideline does not assure code 

compliance. 

Establishing depressurization limits for Category I 

appliances and draft hood-equipped water heaters is 

not yet a part of the appliance certification process, 

so this procedure omits the discussion of 

depressurization limits in the area of the appliance 

until that research has been conducted. 

Combustion appliance testing should be performed under two conditions: with the equipment off 

and then with the equipment running. Minimum steps are listed for the basic trained inspector 

and advanced steps are provided for those with advanced training or for skilled heating, 

ventilation, and air conditioning (HVAC) technicians. If the auditor is not trained in an area, that 

step should not be attempted. 

Check the ambient CO level and combustible gas concentration. If the results of this inspection 

identify a baseline house CO level above the 35 ppm National Renewable Energy Laboratory 

(NREL) Standard Work Specification Limit (https://sws.nrel.gov/spec/202011) or a gas leak 

above the 10% lower explosive limit OSHA standard (OSHA 2011) limit for confined spaces), 

the homeowner should be notified and the inspection should stop until the situation is corrected. 

If the any appliance is going to be replaced in the upgrade step and can be removed from service, 

it need not be tested. If all natural draft appliances are going to be replaced with appliances that 

are not natural draft, and they can be removed from service, then remove them from service and 

proceed with the upgrade in Section 4 of this Guideline. 

3.2.1 On Entry and With Equipment Off

The following tests should be performed with the combustion appliance equipment off: 

Figure 7. Category I furnace

Source: Gas Technology Institute
13 

Minimum Steps

1. Conduct a test for gas leakage. 

2. Visually inspect the venting system to determine that there is no blockage, restriction, 

leakage, corrosion, or other deficiencies that could cause an unsafe condition (NFPA 54 

Chapter 13, IFGC Chapter 5). 

Advanced Steps (for those with proper training) 

1. Identify type of vent connector and common vent materials. 

2. Take basic measurements and check them against the tables in the NFGC and IFGC to 

determine that the vent system is properly sized:

a. Check the vertical common vent/chimney diameter and approximate height and 

note offsets. 

b. Measure the common vent manifold diameter and horizontal length. 

c. Check each appliance vent connector diameter, rise, length, and number of 

elbows. 

3. Shut off all gas to the appliance and shut off any other fuel gas burning appliance within 

the same room. Shut off electrical power at the service disconnect. Use the shutoff valve 

in the supply line to each appliance. 

4. Inspect burners and crossovers for blockage and corrosion. 

5. For furnace installations, inspect the heat exchanger for cracks, openings, or excessive 

corrosion. 

6. For boiler installations, inspect for evidence of water or combustion product leaks. 

If the vent system sizing, installation, or combustion air requirement is not met, the deficiency 

should be noted and addressed after combustion testing in Section 3.2.2 is complete. 

3.2.2 Combustion Appliance Tests With 

Equipment On

The following tests should be performed with the 

combustion appliance equipment running: 

Minimum Steps

1. Close all exterior doors, windows, and fireplace 

dampers. Open all interior doors. Leave open all 

combustion air openings to the outdoors. 

2. If there is a central air handler and a bedroom or 

separate room without a ducted return grille, 

close the door(s) to those rooms. 

Figure 8. Draft hood-equipped 

water heater smoke pen testing

Source: Gas Technology Institute
14 

3. Turn on any clothes dryers. Turn on any exhaust fans, such as range hoods and bathroom 

exhausts, so they will operate at maximum speed. Do not operate a summer exhaust fan. 

Close fireplace dampers. 

4. Place the appliance being inspected in operation. Follow the lighting instructions, if 

required. Adjust the thermostat so the appliance will operate continuously. 

5. Measure the CO level in the flue and determine that the value is not above the ANSI 

certification limit for that type of appliance (Table 4).

6. Test for spillage at the draft hood relief opening after 5 minutes of main burner operation. 

Use the flame of a match or candle or smoke (Figure 8).

Table 4. CO Thresholds

COAFppm = � 20.9

20.9 − O2

� × COppm

COAFppm = �

UCO2

CO2

� × CO

CO THRESHOLDS

(Excerpted from Proposed National Fuel Gas Code 2015 Annex G Ver 2-27-14, printed 

with permission of the American Gas Association)

Appliance Threshold Limit

Central Furnace (all categories) 400 ppm1

air free2,3

Floor Furnace 400 ppm air free

Gravity Furnace 400 ppm air free

Wall Furnace (BIV) 200 ppm air free

Wall Furnace (Direct Vent) 400 ppm air free

Water Heater 200 ppm air free

1 Parts per million

2 Air free emission levels are based on a mathematical equation (involving carbon monoxide and oxygen or carbon 

dioxide readings) to convert an actual diluted flue gas carbon monoxide testing sample to an undiluted air free flue 

gas carbon monoxide level utilized in the appliance certification standards. For natural gas or propane, using 

as-measured CO ppm and O2 percentage: 

Where:

COAFppm = Carbon monoxide, air-free ppm

COppm = As-measured combustion gas carbon monoxide ppm 

O2 = Percentage of oxygen in combustion gas, as a percentage 

3 An alternate method of calculating the CO air free when access to an oxygen meter is not available:

Where:

UCO2 = Ultimate concentration of carbon dioxide for the fuel being burned in percent for natural gas (12.2 

percent) and propane (14.0 percent) 

CO2 = Measured concentration of carbon dioxide in combustion products in percent 

CO = Measured concentration of carbon monoxide in combustion products in percent 
15 

7. Turn on all other fuel gas burning appliances within the same room so they will operate at 

their full inputs. Follow lighting instructions for each appliance. Circulating air blowers 

(if any) should be operating. 

8. Repeat step 6 (Section 3.2.2), above, on the appliance being inspected. 

9. If another appliance is to be inspected, shut off all appliances and return to Step 1 in this 

section (3.2.2). 

10. Return doors, windows, exhaust fans, fireplace dampers, and any other fuel gas burning 

appliance to their previous conditions of use. 

If the combustion appliance tests fail the spillage test with other equipment on but passes under 

natural conditions with other equipment off, the cause and potential improvement is most likely 

with the availability of combustion air, not with the performance of the vent system. Proceed to 

Section 4 and re-evaluate at Test-Out in Section 5. If the combustion appliance fails the spillage 

or combustion tests under natural conditions, a service technician should be called before 

proceeding to upgrade in Section 4 (also see Sections 6 and 7 for remediation). 

Advanced Steps (for those with proper training) 

1. Determine that the pilot(s), where provided, is burning properly and that the main 

ignition is satisfactory, by interrupting and re-establishing the electrical supply to the 

appliance in any convenient manner. If the appliance is equipped with a continuous 

pilot(s), test all pilot safety device(s) to determine whether it is operating properly by 

extinguishing the pilot(s) when the main burner(s) is off and determining, after 3 minutes 

that the main burner gas does not flow upon a call for heat. If the appliance is not 

provided with a pilot(s), test for proper operation of the ignition system in accordance 

with the appliance manufacturer’s lighting and operating instructions.

2. Visually determine that the main burner gas is burning properly (i.e., not floating, lifting, 

or flashing back). Adjust the primary air shutters as required. If the appliance is equipped 

with high and low flame controlling or modulation, check for proper main burner 

operation at low flame. 

3. Furnace installations. Check both the limit control and the fan control for proper 

operation. Limit control operation can be checked by blocking the circulating air inlet or 

temporarily disconnecting the electrical supply to the blower motor and determining that 

the limit control acts to shut off the main burner gas. 

4. Boiler installations. Verify that the water pumps are in operating condition. Test low 

water cutoffs, automatic feed controls, pressure and temperature limit controls, and relief 

valves in accordance with the manufacturer’s recommendations to determine that they are 

in proper operating condition. 
16 

4 Upgrade

Once the house passes the combustion safety requirements at test-in, or provision is made to 

replace the failed appliances, the upgrades can be performed. For inspectors performing the 

minimum test, procede to Section 5 after Upgrade.
17 

5 Test-Out

The next step of the screening process is test-out. The same procedure used in Section 3.2 for 

test-in should be used for test-out. An inspector conducting the minimum combustion safety test 

need not proceed beyond test-out. Failures, if any, should be noted. 

For inspectors with advanced training or HVAC technicians, if it is anticipated that the air 

sealing measures will reduce the infiltration rate below 0.4 ACH, as in Section 2.3.2., the 

combustion air requirements should be rechecked with the known air infiltration rate method and 

adjustments should be made as appropriate. 

Test-out failures are addressed in Section 6, Estimate Remediation Cost, and Section 7, 

Remediation. Remediation is an advanced step that should only be conducted by an inspector 

with proper training or an HVAC technician. 
18 

6 Estimate Remediation Cost

Remediation is an advanced step that should only be conducted by an inspector with proper 

training or an HVAC technician. 

Where the Standard Method in Section 2.3.1, the Known Air Infiltration Rate Method in Section 

2.3.2, the vent testing procedure in Section 3.2, the test-in procedure in Section 3, or the test-out 

procedure in Section 5 fail, remediation will be required to bring the house up to code. Details of 

the remediation measures are covered in Section 7. The remediation cost shoud be estimated 

before remediation is initiated by following the guidance in this section. 

If the remediation cost exceeds the cost of replacing one or both appliances with direct vent 

appliances or an alternative, and no other repairs are possible to provide adequate combustion 

and dilution air to the appliances, appliance replacement should be added to the list of measures 

to be performed on the home. If the draft hood appliances are to be replaced, the remediation 

step associated with combustion safety may be omitted.

6.1 Estimate Remediation Cost at Code Compliance Inspection

If the vent system does not conform to code, it must be replaced before proceeding with the 

upgrade. 

If the inspection fails the Standard Method, or the Known Air Infiltration Rate Method for 

calculating required volume in the space, the ventilation air is insufficient for the Category I or 

draft hood-equipped appliances. In either case, additional volume needs to be added to the space 

communicating with the appliances. A contractor who can perform modifications to the house 

such as adding louvered doors and makeup air, should be consulted for a cost estimate. 

6.2 Estimate Remediation Cost at Test-In

If the house fails at test-in due to appliance combustion or ambient CO level issues, a trained 

HVAC technician will be required to correct the problem before additional work can be 

performed, and that cost should be considered. If the house fails at test-in due to long spillage 

time, a contractor who can perform the remediation steps identified in Section 7.3 should be 

consulted to determine project cost. If the appliances are to be replaced, the house upgrade can 

proceed as long as an unsafe condition is not detected; see Section 3.2, Vent Testing. 

6.3 Estimate Remediation Cost at Test-Out

If the house fails at test-out, due to appliance combustion or ambient CO level issues, a trained 

HVAC technician will be needed to correct the problem. If the house fails at test-out due to long 

spillage time, the remediation measures in Section 7.3 should be considered and a trained HVAC 

technician or contractor should be consulted to determine cost. 

Again, if the remediation cost estimate exceeds the cost of replacing one or both appliances with 

direct vent appliances or an alternative, appliance replacement should be added to the list of 

measures to be performed on the home and the remediation step can be omitted if there are no 

other safety issues identified.
19 

7 Perform Remediation Where Required

Once a cost estimate is complete and remediation has been determined as necessary, the next part 

of the upgrade process is to identify and implement any remediation required to address the 

failed combustion safety testing. Remediation is an advanced step that should only be conducted 

by an HVAC technician with proper training. 

The goal of remediation is to ensure that: 

1. Combustion gases from the appliances are exhausted to the outside through codecompliant vent systems.

2. Adequate combustion air is provided to the appliances. 

3. The installation passes the test-in and test out requirements. 

As indicated in earlier sections of this Measure Guideline, remediation can take place after the 

code compliance inspection, after test-in, after the upgrade, and after test-out. 

7.1 Standard Method Remediation 

If the volume of air for combustion and ventilation using the Standard Method is inadequate and 

makeup air has been provided, additional house air volume needs to freely communicate with the 

appliance(s). Common methods for increasing the volume of air include: 

1. Installing louvers in doors that separate the appliances from other spaces in the house. 

Use the net free area calculation.

2. Providing transfer openings for return air from bedrooms and other areas separated by a 

door from a central return (see Florida Mechanical Code 2010, Section 601.3). 

3. Adding combustion air openings to the outdoors. 

As in Section 6.1, a contractor who can perform these modifications should be consulted to 

determine project cost. If the appliances are to be replaced, the natural draft appliance should be 

turned off and the house upgrade can proceed as long as an inrelated unsafe condition is not 

identified.

7.2 Known Air Infiltration Rate Method Remediation

If the volume of air for combustion and ventilation using the Known Air Infiltration Rate 

Method is inadequate, the ventilation air is insufficient for the Category I or draft hood-equipped 

appliances. Additional volume needs to freely communicate with the appliance as in Section 7.1. 

7.3 Test-in Remediation

If the house fails at test-in due to appliance combustion or ambient CO level issues, a trained 

HVAC technician should correct the problem before additional work can be performed. If the 

house fails at test-in due to long spillage time, the remediation measures in Section 7.1 or Section 

7.2 should be considered first. 
20 

Additional possible remediation measures to be considered at test-in include: 

1. Install louvers in doors that separate the appliances from other spaces in the house or 

replace with full louvered doors. 

2. Seal return air ducts passing through the space housing the appliance. 

3. Seal supply duct leakage to outdoors to less than 6% of total system flow. 

4. Replace high flow intermittent exhaust with low flow continuous exhaust meeting 

bathroom and kitchen requirements. 

5. Provide additional jump ducts in the attic or crawlspace to balance internal operating 

pressures so the mechanical room changes less than 1 Pa to outside during HVAC 

operation. 

6. Provide supply-only ventilation ducted to the air handler return sized for dryer makeup 

air and interlocked with the dryer. 

7. Provide additional ducted returns direct to the air handler fan cabinet from spaces isolated 

from central returns. Increase the size of the central return filter grilles to reduce the face 

velocity to 300–500 ft/minute and reduce operating pressure on the return system. 

8. Supply makeup air as required by the code (NFPA 54 Section 9.3.1.5). Off the shelf 

conditioned makeup air systems are now available for lower flows matching residential 

needs. Intermittent operation adds little to operating cost.

9. Convert from exhaust-only ventilation to balanced ventilation. 

10. Add combustion air openings to the outdoors. 

11. Provide an engineered solution, such as a powered vent. 

Once a remediation path is chosen, a contractor who can perform these modifications should be 

consulted and remediation should take place before the upgrade is performed. If remediation is 

unsuccessful and the appliances are to be replaced, the house upgrade can proceed as long as an 

unrelated unsafe condition is not detected. Details on some of these remediation options are 

included in Section 7.5. 

7.4 Test-Out Remediation

If the house fails at test-out due to appliance combustion or ambient CO level issues, a trained 

HVAC technician will be needed to correct the problem. If the house fails at test-out due to long 

spillage time, the remediation measures in Section 7.3 should be considered. Under no 

circumstances should the house be left in an unsafe condition after test-out. 

7.5 Detail on a Sample of Remediation Options

A summary of the implementation details for several remediation options discussed above is 

provided after the numbered list below (see detailed list in Section 7.3): 

1. Installing louvers in doors that separate the appliances from other spaces in the house 

2. Sealing return air ducts passing through the space housing the appliance 
21 

3. Supplying makeup air as required by the code (NFPA 54 Section 9.3.1.5) 

4. Adding combustion air openings to the outdoors 

5. Converting from exhaust-only ventilation to balanced ventilation 

6. Providing an engineered solution, such as a powered vent. 

7.5.1 Installing Louvers in Doors That Separate the Appliances From Other 

Spaces in the House

Openings used to connect indoor spaces shall be sized as follows (NFPA 54 Section 9.3.2.3). 

Note that an open door is not considered an opening. 

1. Combining spaces on the same story. Each opening shall have a minimum free area of 1 

in.2

/1,000 Btu/h of the total input rating of all appliances in the space but not less than 

100 in2

. One opening is required at the top and one at the bottom of the enclosure and the 

minimum dimension of either opening is 3 in. 

2. Combining spaces on different stories. The volume of spaces on different stories shall be 

considered as communicating spaces where such spaces are connected by one or more 

openings in doors or floors having a total minimum free area of 2 in.2

/1,000 Btu/h of the 

total input rating of all the appliances.

The required size of the opening shall be based on the net free area of louvers or grilles covering 

the openings. Where the net free area is unknown, use 25% for wood louvers, 75% for metal 

louvers, and 75% for metal grilles. Louvers and grilles shall be fixed in the open position. 

7.5.2 Sealing Return Air Ducts and the Furnace Cabinet

7.5.2.1 Seal Joints

Seal all joints, penetrations, and openings in the cabinets of the air handling unit. Use metal tape 

or suitable gaskets on service openings and permanent seals on fixed joints and seams. All air 

handling units should be mechanically attached to other air distribution system components. 

Provide continuous sealed ductwork from the living space directly into the blower housing for 

the forced air furnace or other air handler. Maintain at least the cross sectional area of the return 

inlet and size the liner or duct to meet the manufacturer’s required airflow without restriction. 

Furnace returns without the manufacturer’s information must provide at least 2 in.2

 of total cross 

sectional area for every 1,000 Btu of output. (See Proctor and Chitwood 2011.) 

Mechanically fasten the duct to the furnace cabinet and at the connection to the interior. Tape all 

cabinet service openings and joints. Seal all joints and seams with mastic and mesh tape meeting 

UL181-M, or foil tapes meeting Underwriters Laboratories (UL) 181 A-P and UL 181 B-FX. 

Sealant foam approved for use as a firestop in combustible construction is acceptable at the 

connection to the interior construction materials. 

No return air can be taken from a garage, boiler room, furnace room, or unconditioned attic. Line 

any enclosed support platform or building cavity with continuous, durable, air impermeable 

material and seals that show a flame spread of 25 or less and smoke developed of 50 or less 
22 

when tested to ASTM E 84 or UL 723 or use a standard material listed for use in duct systems, 

i.e., sheet metal, duct board, flexible duct. 

7.5.2.2 Visual Inspection of Air Handler

Visually inspect to verify that the following locations have been sealed. These include: 

1. Seams, joints, and openings at the fan cabinet, furnace cabinet, the cabinet section with 

the air conditioner coil, and connections to return and supply plenums 

2. Refrigerant line and other penetrations into the forced air unit 

3. Tape or adjust approved gasket at air handler door and other service panels (no 

permanent sealants)

4. Plenum and duct seams 

5. Return plenum joints at filter grille and to the back side of interior finish materials

6. Gap between any duct and duct chase where it passes out of the combustion space.

7.5.2.3 Smoke Test of Duct and Cabinet Leakage in Appliance Area

Perform a smoke test to confirm completion or compliance of duct and cabinet sealing work: 

1. With the furnace fan and service disconnect off, set up for fan pressurization of the duct 

system for the furnace or air handler located in the combustion space. Close registers to 

limit smoke entry to the interior. Seal the grilles and registers necessary to reach +25 Pa 

inside the ducts in the specific combustion space. 

2. With the duct pressurization fan running, inject theatrical fog or equivalent nontoxic 

smoke into the fan inlet while observing smoke coming out of the ducts and cabinet. 

Mark the spots and stop adding smoke. 

3. Seal all joints, penetrations, and seams where smoke came out, use foil tape or approved 

gasket on service openings in the cabinet, and place permanent seals on all other 

openings. 

4. Inject additional smoke and inspect the seals. If smoke continues to come out of the 

return plenum or cabinet, turn off the pressurization fan and open the air handler fan 

cabinet to access and seal the remaining return leaks in the combustion space.

5. There must be no return leaks. Return air from outside the combustion space must be 

brought to the furnace fan through continuous airtight ducts. 

6. The furnace duct section is sealed when smoke no longer comes out of any openings in 

the combustion space. 

Note any smoke coming out of the burner area, heat exchanger, or vent connector for follow up 

service. For further information refer to the NREL Standard Work Specifications Tool 

(https://sws.nrel.gov/spec/316025).
23 

7.5.3 Supplying Makeup Air as Required by the Code (NFPA 54 Section 9.3.1.5)

Where exhaust fans, clothes dryers, and kitchen ventilation fans interfere with the operation of 

appliances, makeup air is required. (See Section 7.5.4.) 

7.5.4 Adding Combustion Air Openings to the Outdoors

Outdoor combustion air or a combination of indoor and outdoor combustion air is permitted by 

the code when openings are sized appropriately. The size of openings to the outdoors are based 

on appliance input and adjusted by the indoor volume communicating with the appliances. See 

NFPA 54 Sections 9.3.3 and 9.3.4. 

7.5.5 Converting From Exhaust-Only Ventilation to Balanced Ventilation

In homes that employ exhaust-only ventilation, a balanced ventilation solution such as an air-toair heat exchanger or enthalpy exchanger can be used. For purposes of combustion safety, a less 

costly option is to supply makeup air as in Section 7.5.3. 

7.5.6 Providing an Engineered Solution

A professional engineer can perform an assessment of the installation and determine if a powered 

vent system or other approach can be used for remediation. 
24 

8 Materials for Duct Sealing

Use materials and sealants that match the listing where they are applied:

• Tapes for metal ductwork marked 181 A-P 

• Mastic that meets 181 A-M 

• Non-metallic flexible duct: 

• Tape and clamping system 

• Mastic that meets UL 181 B-M 

• Mechanical fasteners labeled 181 B-C 

• Aerosol sealant with 25/50 class 1 flame spread is approved for all joint seals when 

applied by manufacturer certified installers to manufacturer’s standards

• Duct coverings, linings, and adhesives flame/smoke 25/50 on ASTM E 84 or UL 723 

• Two-part spray foam meeting IRC 2012 M1601.4.1, and IRC R316.6 tested for its 

intended uses to: NFPA 286 within acceptance criteria of IRC R 302.9.4, FM 4880, UL 

1040, or UL 1715 and reported in an International Code Council Evaluation Service 

report as meeting requirements of code, requires no additional sealants. 

• Single component sealant foam: use material accepted as firestop in wood frame 

construction and approved for limited exposed use. 
25 

References

BPI 1200 Basic Analysis of Buildings, 2012, Building Performance Institute, Malta New York. 

Department of Energy, 2013, Energy Efficiency and Renewable Energy, undated, “Venting 

Issues I Gas-Fired Water Heaters”, 

website: http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/d-3.pdf . 

Fitzgerald, J.; Bohac, D. (2014). Isolated Combustion Appliance Zone Testing. Building America 

Report for the National Renewable Energy Laboratory (not yet published). 

ICC. (2010). “Florida Building Code, Mechanical.” International Code Council. 

ICC. (2013). International Fuel Gas Code. International Code Council. 

IRC. (2013). International Residential Code for One and Two Family Dwellings, International 

Code Council. 

National Renewable Energy Laboratory Standard Work Specifications for Home Energy 

Upgrades, http://www.nrel.gov/tech_deployment/buildings_sws.html. 

NFPA. (2012a). National Fire Protection Association Standard 54). National Fuel Gas Code,

National Fire Protection Association, American Gas Association, Boston, MA. 

NFPA. (2012b). National Fire Protection Association Standard 211. Standard for Chimneys, 

Fireplaces, Vents, and Solid Fuel-Burning Appliances.

Proctor, J.; Chitwood, R.; Wilcox, B.A. (2011). “Efficiency Characteristics and Opportunities for 

New California Homes.” California Energy Commission. Publication number: CEC 500-2012-

062. 
DOE/GO-102014-4380 ▪ April 2014

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.

buildingamerica.gov